We recently tested some metallic powders on our large metal cabinets. The surface is all splotchy with some dark areas and some very shiny areas. We use manual, corona charging spray guns. Why is the surface so inconsistent in color and appearance?

Metallic powders available on the market can be divided into two broad categories: "bonded" and "dry-blended." In a bonded powder, the metallic flakes are attached (bonded) to conventional powder particles (those combined of resin, pigment, various additives, etc.) to form a rather homogenous mixture--a mixture in which all particles have similar specific gravity, size, dielectric properties, shape, etc. For the most part, these bonded metallic powders handle and apply just as regular, nonmetallic materials.

The difficulties described in the question are often observed with dry-blended metallic powders. Especially when they are applied manually. The difficulties arise from the nonhomogeneous nature of the mix. The differences between the components combining the dry-blended powder mixture are of a dual nature-aerodynamic and electrostatic.

First, metallic particles (flakes) have different specific gravity, shape, mass and size compared to those of conventional powder particles. Thus, metallic particles act differently when propelled by the air stream from a spray gun--aerodynamic differences.

Depending on a particular formulation and milling, the metallic particles can segregate from "resin" particles within the spray pattern and, for example, be more concentrated in the peripheral areas of the spray. Additionally, having a different mass and shape, they respond differently to the changes in the direction of the spray pattern and to the air turbulences within recesses of a part.

For example, the metallic particles can "lag behind" as an operator moves the spray gun along the part's surface. The difference in the way metallic particles respond to the changes in spray pattern direction and velocity is often the first contributor to the blotchy appearance of the finish when dry-blended metallic powders are applied manually.

Second, metallic particles have dramatically different "electrostatic" properties compared to conventional powders. They charge differently, orient themselves differently in the electric field, and can often position themselves differently on the part's surface, depending on the electric field strength.

For example, if an operator moves the gun too close to the part, those metallic flakes that are deposited on the part's surface can stand up on their edge, aligning themselves with the electric field. It is important to keep in mind that the metallic appearance of the finished part depends on the angle at which the light reflects off the metallic particles embedded in the finish. If those particles stand up on their edges (lining up with the electric field) the reflectivity will be lost and the finish will appear dull or dark compared to the rest of the part.

What to do here? First, consider using bonded metallic powders. They are slightly more expensive, but the reduction in your rework and quality costs are likely to cover the extra cost of powder.

Second, try automating your powder coating process as much as possible. Automation eliminates the variables introduced in manual coating and typically yields more consistent and repeatable results.

Third, train the operator to minimize the changes in the spray gun velocity as it is passed over the part's surface and in the gun-to-part distance.

Fourth, if your equipment has adjustable current-limiting capability, set the current limit to 10 to 15 mA. This will minimize fluctuations in the electric field strength during application. If your spray system does not have current limiting, lowering gun-tip voltage often improves finish uniformity when spraying dry-blended metallic powders.

Sergey Guskov is a powder systems specialist at Nordson Corp.

Spray Booth Material Differences

My company is considering one of those new systems designed for quick color change. We obtained quotes from several vendors and noticed that the material of which the spray booths are made differs from vendor to vendor. How important is the booth material and which material is better?

In the attempt to reduce color change time in modern powder coating systems, vendors evaluate multiple booth-canopy materials. Recommending one material over another is not appropriate here. A better approach is to help you understand the impact of material selection on your system's performance and to provide you with questions you can ask your vendors.

Powder attraction to the booth walls. Whether the booth is made of double-wall materials, solid plastic or composite dielectric material, the electric field will penetrate it, like light goes through glass. There is no difference in the way double-wall ("sandwich" construction) material works in the presence of the electric field compared to a solid or composite material. However, if there is a metal support structure near the outside booth wall, the electric field will concentrate on it, resulting in more intense powder deposition on the inside of the booth in that area. Make sure there are no metal hardware or support structures touching or near the booth walls.

The ease of powder release from booth walls. No matter the material, some powder will deposit on the booth walls. The color change time then will depend on how easy it is to blow that powder off. The ease with which powder is released from the booth walls is the principal difference between available booth materials. Ask your equipment vendors to conduct a demonstration in their test facilities and judge for yourself which material cleans better.

Sustainable performance. With time, any booth material will accumulate water vapor, residual charge, oils (from being touched by hands) and other contaminants on its walls and will require cleaning/conditioning to restore its original performance. Some materials require the use of chemicals to restore their original performance; others, water or a mild soap solution. Talk to your equipment vendors about recommended cleaning materials, and don't forget to visit systems that have been in operation for some time and talk to the people who work with them.

Ease of damage/repair. The booth materials available on the market vary in their strength, impact and scratch resistance, and ease of repair. Ask your vendors about repair procedures if the booth material gets scratched or broken. How long does it take to replace or repair panels inside the booth? What is the delivery time on replacement panels? What's the impact resistance?

Presence of seams inside the booth. Even well-sealed seams, with time, create the potential for powder entrapment. Thus, the fewer the number of seams on the internal booth surfaces, the better. The number of seams will depend on the booth size and the manufacturing process used to construct booth panels. Ask your vendor about booth construction, and the presence and number of seams.